To gain more power, use your back leg for resistance and drive.
The back leg plays an important part during the swing for driving the swing forwards and generating power. It plays two parts in the swing: Generating resistance through a big wind up in the backswing, and driving the body weight and club through the ball in the forward part of the swing.
At the set up position to the golf ball, check that the back leg is set slightly bent at the knee and maintain the same angle in the bend of the leg throughout the back swing. The set position of the back leg allows the hips and shoulders to turn against the knee, stretching the muscles in the mid-section of the body, and providing resistance to allow the backswing to generate it's coil.
This action promotes more speed in the forward part of the swing as those stretched muscles are released and unwound.
A good exercise is to feel that the back knee remains over the toes in the back swing. Take a practice swing and swing back holding the position at the top of the backswing. Check that the knee is still bent slightly and that the knee points out over the toes of the back foot. If this is the case there should be tension across the side of the back knee and the back hip.
In the downswing, the right leg should drive forwards into the ball pushing up off the floor so that the whole of the right side of the body attacks the ball at the same time as the golf club. When this happens, the golfer achieves maximum power as the whole of the body is driving into the ball rather than just the upper body.
To practice driving forwards, take some loose practice swings and when swinging through the ball, feel the back knee and the back trouser pocket arrive at the impact area at the same time as the hands. The sensation is one of pushing off the ground with the back foot, lifting the heel off the floor as the club approaches the ball. When timed correctly, the speed of the club increases immeasurably through the ball.
Use these two exercises to increase your driving distance.